Multi-beam exposure apparatus

ABSTRACT

The invention is structured such as to improve a multi-beam exposure apparatus for scanning a plurality of beams, and provides an exposure apparatus in which no color shifting or no shading and no spread of a line is generated by reducing a mutual shifting of the respective beams so as to accurately overlap an image. A multi-beam exposure apparatus according to the invention has optical elements on the middle of an optical path between an image forming lens disposed between an optical deflecting apparatus and an image surface, and a light detector for detecting a horizontal synchronism, the optical elements being structured such as to change an emitting angle in correspondence to a change of a wavelength due to a temperature change of a laser beam from a light source with respect to a main scanning direction and to shift a beam position at the same amount as a position shifting amount generated by an image forming lens according to a difference of a wavelength and in an opposite direction thereto, thereby guiding the beam to the same position on a detecting surface of the light detector for detecting the horizontal synchronism even in the case that wavelengths of the light beams from the light sources which emit two laser beams at every colors separated into color components change according to a change of the temperature.

TECHNICAL FIELD

[0001] The present invention relates to a multi-beam exposure apparatusscanning a plurality of light beams and usable for a plural drum typecolor printer apparatus, a plural drum type color copier, a multicolorprinter, a multicolor copier, a monochromatic high speed laser printer,a monochromatic high speed digital copier and the like.

BACKGROUND ART

[0002] For example, in an image forming apparatus such as a colorprinter apparatus, a color copying apparatus which employs an imageforming unit including a plurality of photosensitive drums, there isemployed an exposure apparatus which supplies a plurality of light beamshaving the number the same as the number of plural image datacorresponding to separated color components, that is, at least thenumber of image forming units.

[0003] This kind of exposure apparatus has a plurality of semiconductorlaser elements emitting a predetermined number of light beamscorresponding to the image data at each of the separated colorcomponents, a first lens group stopping down a cross sectional beamdiameter of the light beam after each of the semiconductor laserelements is emitted to a predetermined size and shape, an opticaldeflecting apparatus operated in accordance that a recording mediumholding the image formed by each of the light beams continuouslyreflects in a direction perpendicular to a transferred direction, asecond lens group image forming the light beam deflected by the opticaldeflecting apparatus on a predetermined position of the recordingmedium, and the like. Here, in many cases, a direction in which thelaser beam is deflected by the optical deflecting apparatus is indicatedto be a main scanning direction, and a direction in which the recordingmedium is transferred, that is, a direction perpendicular to the mainscanning direction is indicated to be a sub scanning direction.

[0004] The exposure apparatus mentioned above is classified into anembodiment which employs a plurality of exposure apparatusescorresponding to each of the image forming units according to theapplied image forming apparatus and an embodiment which employs amulti-beam exposure apparatus capable of supplying a plurality of lightbeams by means of one exposure apparatus. In this case, in recent days,in order to increase an image forming speed and improve a resolution,there has been also proposed a high speed printer apparatus capable offorming an image having a high resolution and forming an image at a highspeed by exposing image data having the same color in a parallel manner.

[0005] However, in the exposure apparatus mentioned above, when arotational speed of the reflection surface of the optical deflectingapparatus is increased in order to increase the speed of the imageforming apparatus and improve the resolution, it is necessary to employan expensive bearing such as an air bearing which can stand against thehigh speed rotation on the reflection surface. On the other hand, aswell as the rotational speed of the motor has an upper limit, the motorcapable of rotating at a high speed is expensive and a drive circuit forrotating the motor is also expensive, so that there is a problem that anincrease of the rotational speed of the reflection surface correspondingto an increase of the cost can not expected. In this case, when therotational speed of the reflection surface is increased, as a result,there is generated problems that the wind sound is increased as well asthe wind damage on the reflection surface is accelerated.

[0006] On the other hand, it is possible to restrict an increase of therotational speed of the motor when increasing the number of thereflection surfaces, however, since an image frequency is increased,there is a problem that a noise component superimposed on the imagesignal (image data) at a high possibility is increased. Further, whenthe image frequency is increased, there is a problem that various kindsof limitations are generated in designing or mounting the controlcircuit.

[0007] Accordingly, there has been proposed a multi-beam exposure whichcan reduce each of the rotational speed of the reflection surface andthe image frequency by allocating a plurality of light beams to each ofthe separated color components and deflecting (scanning) them at onetime, however, even in the case of employing the multi-beam exposure,there are various kinds of problems as mentioned below.

[0008] That is, in the multi-beam exposure, there is employed a methodof using a plurality of light sources for each of the separated colorcomponents and combining the light beams emitted from each of the lightsources at a color component unit so as to deflect (scan) as one lightbeam, and a semiconductor laser element is employed for the lightsource.

[0009] However, it has been known that a wavelength of the light beam(the laser beam) irradiated from the semiconductor laser element isvaried in a luminescent wavelength according to a temperature of anenvironment in which the laser element is placed. Further, each of thesemiconductor laser elements is different in a changing amount of theluminescent wavelength with respect to the temperature change. In thiscase, when the temperature is varied in the periphery of each of thesemiconductor laser elements and levels of a change with age aredifferent from each other in each of the laser elements, the wavelengthsof the light beams emitted from the respective light sources are varied.

[0010] Further, since the characteristic of the semiconductor laserelement includes a mode hopping phenomenon that the luminescentwavelength is about 1.5 nm changed with respect to the temperaturechange about 0.1° C., it is hard to uniformly align the luminescentwavelength of all the laser elements at a wide environmental temperaturerange even when aligning the luminescent wavelength under a certaincondition.

[0011] As mentioned above, in the case that the luminescent wavelengthof the light beam irradiated from each of the semiconductor laserelements is changed due to the temperature change, in detection of ahorizontal synchronism and definition of a writing position structuredsuch as to arrange a beam detecting sensor for detecting the horizontalsynchronism, for example, at a position equivalent to a mirror surface,to detect the fact that the light beam enters into the sensor byemitting the light beam prior to a timing at which the beam passesthrough the sensor, and to write the image by setting that the lightbeam is at the same position at the detecting timing and enters into theimage area after a fixed time thereafter, an oscillating angle when thelight beam is guided to each of the reflection surfaces of thedeflecting apparatus becomes a different angle even when the timing atwhich the light beam enters into the sensor is the same.

[0012] That is, since the position of the beam detecting sensor isfixed, the writing position of the image is substantially maintained toa fixed value when writing the image a fixed time after detecting thefact that the light beam enters the sensor in the case that theluminescent wavelength is changed due to the temperature change,however, at a position opposite to the writing position, at which theexposure of the image is finished, there is a problem that a twice timesof change (2×Δθ) is generated when setting the change amount of theoscillating angle on each of the reflection surfaces in the deflectingapparatus when the light beam scanned to the same place by the changecomponent of the wavelength changing due to the change of thetemperature reaches, to Δθ.

[0013] This generates a phenomenon that a color is shifted, apredetermined color can not reproduced and the like, in a color printerapparatus, and there is a problem of reduction of the resolution andgeneration of jitter caused by changing an outer diameter and a shape ofa dot (an assembly of the light beam on the photosensitive body)constituting the image, in a high speed printer apparatus.

DISCLOSURE OF INVENTION

[0014] An object of the present invention is to provide an exposureapparatus for scanning a plurality of beams, in which difference betweenthe respective beams is reduced and an image is accurately formed,thereby preventing a color shifting or a reduction of resolution fromgenerating.

[0015] According to the present invention, there is provided amulti-beam exposure apparatus comprising a plurality of light sourcesfor irradiating light beams having predetermined wavelengths,pre-deflection optical means for applying a predetermined opticalcharacteristic to the light beam irradiated from each of the lightsources, deflection means for deflecting the light beam passing throughthe pre-deflection optical means to a first direction corresponding to adirection in which a rotatably formed reflection surface is rotated at apredetermined speed, image formation optical means for continuouslyimage forming the light beams deflected in the first direction by thedeflection means on an image surface, detecting means for detecting atleast one of the light beams passing through the image formation opticalmeans and outputting predetermined signals corresponding to the lightbeams, and optical elements arranged between the deflection means andthe detecting means and changing an emission angle in correspondence toa change of a wavelength of the light irradiated from each of aplurality of light sources.

[0016] Further, according to the present invention, there is provided amulti-beam exposure apparatus comprising a plurality of light sources, afirst optical element for assembling light beams irradiated from theplurality of light sources to one light beam so as to give apredetermined characteristic, deflection means for deflecting the lightbeams supplied from the first optical element to a first directioncorresponding to a direction in which the reflection surface is rotated,a second optical element extended out along the first direction andimage forming the light beams deflected from the deflection means to apredetermined position so as to satisfy a function corresponding to arotation of the reflection surface in the deflection means, detectingmeans arranged at a distance optically equivalent to a position at whichthe light beams passing through the second optical element reaches andin an area except an image area in which the light beam passing throughthe second optical element functions as an image and detecting at leastone of the light beams passing through the second optical element so asto output a predetermined signal, and optical elements arranged betweenthe second optical element and the detecting means, changing an emissionangle in correspondence to a change of a wavelength of the lightirradiated from the plurality of light sources due to a change of atemperature with respect to the first direction and shifting a positionto which the light reaches to a direction having the same amount as aposition shifting amount on the predetermined image surface generated bythe second optical element due to a difference of the wavelength andhaving an opposite direction, thereby guiding the light having awavelength which is changed due to a change of the temperature to thesame position on the detecting surface of the detecting means in thecase that the reflection surface of the deflection means has the samerotational angle.

[0017] Still further, according to the present invention, there isprovided a multi-beam exposure apparatus comprising a plurality of lightsources for irradiating lights having a predetermined wavelength at apredetermined temperature, pre-deflection optical means for assemblinglights irradiated from the light sources to one light beam so as to givea predetermined characteristic, deflection means for deflecting a groupof lights emitted from the pre-deflection optical means to a firstdirection, an image formation lens extended out in the first directionand image forming the lights deflected by the deflection means on apredetermined image surface at a uniform speed, detecting means definedat a distance optically equivalent to the predetermined image surface,arranged at a position in which the lights passing through the lensreaches and in an area except an image area among the predeterminedimage surface and detecting the lights passing through the lens so as tooutput a predetermined signal, and optical elements arranged on anoptical path between the lens and the detecting means, changing anemission angle in correspondence to a change of a wavelength of thelight irradiated from the plurality of light sources due to a change ofa temperature with respect to the first direction and shifting aposition to which the light reaches to a direction having the sameamount as a position shifting amount generated by the lens due to adifference of the wavelength and having an opposite direction, therebyreducing a difference of the image of a main scanning direction positionat a position opposite to a writing position in the first directiongenerated by a difference of the writing timing caused by the lightbeing different from a reference wavelength in the wavelength.

BRIEF DESCRIPTION OF DRAWINGS

[0018]FIG. 1 is a schematic plan view showing an embodiment of amulti-beam exposure apparatus corresponding to an embodiment of thepresent invention;

[0019]FIG. 2 is a schematic view showing a state of the exposureapparatus shown in FIG. 1 viewed from a side portion;

[0020]FIG. 3 is a schematic view showing a structure of a lens holderand a light source used for the exposure apparatus shown in FIGS. 1 and2;

[0021]FIG. 4 is a schematic view explaining a mechanism for holding ahalf mirror, a half fixed mirror and a color combination mirror in theexposure apparatus shown in FIGS. 1 and 2;

[0022]FIG. 5 is a schematic view in the case of an optical pathcorrection element employed for the exposure apparatus shown in FIGS. 1and 2 viewed from a cross sectional direction with respect to a lightincidental surface;

[0023]FIG. 6 is a graph showing a state in which a luminescentwavelength is changed when an environmental temperature is changed dueto a mode hopping of the semiconductor laser element;

[0024]FIG. 7 is a schematic view explaining a positional relationshipbetween parameters shown in Table 1, that is, θ1, θ2, D3, θ4, D6, θ7 andy7 in order to determine an angle α to be defined at a time of enteringa laser beam into a prism shown in FIG. 5;

[0025]FIG. 8 is a graph showing a change of a position of a laser beamimage formed on the image surface after passing through a two-assembledlens as a relative position in the main scanning direction in the casethat the wavelength of the laser beam after emitting the laser componentis changed in order to specify a characteristic of the prism shown inFIG. 5;

[0026]FIG. 9 is a graph showing a change of the beam position imageformed on the image surface as a relative position in the main scanningdirection in the case that the wavelength of the laser beam from each ofthe laser components indicates the temperature-wavelength change shownin FIG. 8 in a state of taking out the prism from the multi-beamexposure apparatus shown in FIGS. 1 and 2;

[0027]FIG. 10 is a graph showing an oscillation angle of a reflectionangle of an optical deflection apparatus at a time of detecting a laserbeam having a wavelength of 675 nm and an oscillation angle of thereflection angle of the optical deflection apparatus at a time ofdetecting a laser beam having a wavelength of 680 nm when setting aposition for detecting the laser beam at the position in the mainscanning direction to −160 mm, and showing that a difference of arotational angle Δθ is generated at a degree Δθ=7.5 μrad;

[0028]FIG. 11 is a graph showing a degree of difference in the positionof the laser beam in the main scanning direction opposite to the writingposition at a time of emitting the laser beam when a fixed time haspassed after setting a position for detecting the laser beam in the mainscanning direction position to −160 mm;

[0029]FIG. 12 is a plan schematic view showing another embodiment of themulti-beam exposure apparatus shown in FIGS. 1 and 2; and

[0030]FIG. 13 is a schematic view showing an example of characteristicsof diffraction gratings assembled in the multi-beam exposure apparatusshown in FIG. 12.

BEST MODE OF CARRYING OUT THE INVENTION

[0031] Hereinafter, embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

[0032]FIGS. 1 and 2 are schematic views which show a multi-beam exposureapparatus according to an embodiment of the present invention andassembled in an image forming apparatus for forming a color image on thebasis of color separated image information, for example, separated intofour color components, in which FIG. 1 is a schematic plan view in astate of removing a cover and FIG. 2 is a schematic view showing a statein the case of viewing the apparatus shown in FIG. 1 from a sideportion. In this case, in the color image forming apparatus employingthe image information corresponding to four colors as mentioned above,in order to display an optional color according to a subtractivemixture, since there is generally employed four kinds of imageinformation separated into each of colors comprising yellow (Y), magenta(M), cyan (C) and black (B, however, the black is used for inkingperformed by replacing an image area displaying a black obtained byoverlapping the yellow, the magenta and the cyan by a single color andfor forming a black C monochromatic image such as a document or thelike) and A; four sets of various mechanisms for forming the image ateach of the color components in correspondence to each of Y, M, C and B,the structure is made such as to identify the image information at eachof the color components and the corresponding mechanism by adding Y, M,C and B to each of reference numerals. Further, since an image formingapparatus main body, a control portion thereof (for controlling anoperation), a treatment of the image information signal and the likehave been already disclosed in U.S. Pat. No. 5,715,078 which is a priorapplication filed by the same inventors of the present application andwas already registered (date of registration is Feb. 3, 1998), thedetailed description thereof will be omitted in this specification.

[0033] A multi-beam exposure apparatus 1 has light sources 3Y, 3M, 3Cand 3B for respectively outputting light beams toward four image formingportions of an image forming apparatus main body (not shown), and anoptical deflecting apparatus 7 as deflecting means for deflecting(scanning) the light beams irradiated from the respective light sources3 (Y, M, C and B) toward an image surface arranged at a predeterminedposition (that is, a photosensitive drum provided in each of four imageforming portions of the image forming apparatus main body (not shown))at a predetermined linear speed. In this case, a predeflection opticalsystem 5 is arranged between the optical deflecting apparatus 7 and thelight source 3 and a post-deflection optical system 9 is arrangedbetween the optical deflecting apparatus 7 and the image surface (notshown), respectively. Further, a direction in which the laser beam isdeflected (scanned) by the optical deflecting apparatus 7 is indicatedas a main scanning direction, and a direction perpendicular to each ofthe main scanning direction and an axis corresponding to a reference fora deflecting operation which the optical deflecting apparatus applies tothe laser beam so that the laser beam scanned (deflected) by the opticaldeflecting apparatus becomes in the main scanning direction is indicatedas a sub scanning direction. Accordingly, the sub scanning direction ofthe laser beam deflected by the multi-beam exposure apparatus 1 is adirection in which a recording sheet is transferred in the image formingapparatus main body (not shown), and coincides with a direction in whicha photosensitive drum (not shown) is rotated. Further, the main scanningdirection corresponds to a direction perpendicular to a direction inwhich the recording sheet (not shown) is transferred (an axial directionof the photosensitive drum (not shown)).

[0034] The respective light sources 3 (Y, M, C and B) are structuredsuch that two semiconductor laser elements 3Ya and 3Yb, 3Ma and 3Mb, 3Caand 3Cb, and 3Ba and 3Bb are placed in a predetermined arrangement atrespective separated color components Y (yellow), M (magenta), C (cyan)and B (black).

[0035] The pre-deflection optical systems 5 are structured such as to becombined to one beam by means of group combining mirrors 15Y, 15M, 15Cand 15B for combining laser beams LYa and LYb, LMa and LMb, LCa and LCb,and LBa and LBb respectively emitted from the lasers 3Ya and 3Yb, 3Maand 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb corresponding to the respectivelight sources into one laser beam at the respective color components andto be further combined to substantially one laser beam L {(Lya+Lyb)=LY,(LMa+LMb)=LM, (LCa+LCb)=LC and (LBa+LBb)=LB} with respect to the mainscanning direction by respective color combining mirrors 19M, 19C and19B, thereby being guided toward the light deflecting apparatus.

[0036] In this case, finite focal lenses 13, aperture stops 14 andcylinder lenses 17 are provided between the light sources 3 (Y, M, C andB) and the group combining mirrors 15 (Y, M, C and B), however, sinceeach of the finite focal lenses 13, the aperture stops 14 and thecylinder lenses 17 has been already disclosed in U.S. Pat. No. 5,734,489(date of registration is Mar. 31, 1998) which is a prior applicationfiled by the same inventors of the present application and was alreadyregistered, the detailed description will be omitted in thisspecification.

[0037] The optical deflecting apparatus 7 has a polyhedral mirror mainbody 7 a, for example, in which eight flat reflection surfaces (flatreflection mirrors) are arranged in a regular polygonal shape, and amotor 7 b for rotating the polyhedral mirror main body 7 a in the mainscanning direction at a predetermined speed. Further, the polyhedralmirror main body 7 a is integrally formed with a rotary shaft of themotor 7 b. In this case, with respect to the optical deflectingapparatus 7, since the explanation has been in detail disclosed in U.S.Pat. No. 5,734,489 (date of registration is Mar. 31, 1998) previouslymentioned, the detailed description will be omitted in thisspecification.

[0038] The post-deflection optical system 9 has a two-assembled imageformation lens 21 for optimizing a shape and a position of the laserbeam L (Y, M, C and B) deflected (scanned) by the rotary polyhedralmirror 7 a of the optical deflecting apparatus 7 on the image surface,that is, first and second image formation lenses 21 a and 21 b, anoptical detector for a horizontal synchronism 23 which detects each ofthe laser beams L so as to align the horizontal synchronism of therespective laser beams L (Y, M, C and B) after being deflected in theoptical deflecting apparatus 7 and passing through the two-assembledimage formation lens 21, a reflecting mirror 25 for a horizontalsynchronism which reflects the respective laser beams L toward theoptical detector 23 for the horizontal synchronism, an optical pathcorrection element 27 arranged between the reflecting mirror 25 and theoptical detector 23 for detecting the horizontal synchronism and capableof substantially coinciding the respective laser beams L reflectedtoward the optical detector 23 for detecting the horizontal synchronismby the reflecting mirror 25 with incidental positions on a detectingsurface of the optical detector 23 for detecting the horizontalsynchronism even in the case that the wavelengths of the respectivelaser beams L are changed due to the change of the temperature of theportion (environment) in which the laser elements (the light sources 3(Y, M, C and B)) are arranged, a plurality of mirrors 33Y (yellow No.1), 35Y (yellow No. 2), 37Y (yellow No. 3), 33M (magenta No. 1), 35M(magenta No. 2), 37M (magenta No. 3), 33C (cyan No. 1), 35C (cyan No.2), 37C (cyan No. 3) and 33B (black for exclusive use) which guide therespective laser beams L (Y, M, C and B) emitted from the second imageformation lens 21 b of the two-assembled image formation lens 21 to thephotosensitive drums (the image surfaces) (not shown) corresponding tothe respective laser beams L, and dust-proof glasses 39 (Y, M, C and M)which perform a dust-proof of the optical scanning apparatus 1 includinga lot of optical elements mentioned above. In this case, since reasonsaffecting optical characteristics such as the optical characteristics,the positional relation and the like of each of the two-assembled imageformation lens 21 (the first lens 21 a and the second lens 21 b), theoptical detector 23 for the horizontal synchronism, the reflectingmirror 25 for the horizontal synchronism, the respective mirrors 33Y, 35(Y, M and C), 37 (Y, M and C) and 33B, and the respective dust-proofglasses 39 (Y, M, C and M) mentioned here are common to those of U.S.Pat. No. 5,734,489 (date of registration is Mar. 31, 1998) previouslyshown, the detailed description will be omitted in this specification.

[0039] Next, a description will be given in detail of the pre-deflectionoptical system 5 between each of the lasers 3Ya, 3Yb, 3Ma, 3Mb, 3Ca,3Cb, 3Ba, 3Bb constituting the respective light sources 3 (Y, M, C andB) and the optical deflecting apparatus 7 at each of the lasers.

[0040] The respective light sources 3Y, 3M, 3C and 3B also have a yellowNo. 1 laser 3Ya and a yellow No. 2 laser 3Yb for emitting the laser beamLY, a magenta No. 1 laser 3Ma and a magenta No. 2 laser 3Mb for emittingthe laser beam LM, a cyan No. 1 laser 3Ca and a cyan No. 2 laser 3Cb foremitting the laser beam LC, and a black No. 1 laser 3Ba and a black No.2 laser 3Bb for emitting the laser beam LB, respectively. In this case,the laser beams LYa and LYb, LMa and LMb, LCa and LCb, and LBa and LBbwhich are respectively emitted from the lasers 3Ya, 3Yb, 3Ma, 3Mb, 3Ca,3Cb, 3Ba and 3Bb constituting the respective light sources arerespectively combined by group combining mirrors (half mirrors, that is,the first combining mirrors) 15Y, 15M, 15C and 15B which reflect about50% of the incidental laser beams and transmit about 50% thereof at thesame color component, and combined by color combining mirrors (secondcombining mirrors) 19M, 19C and 19B, thereby being guided toward theoptical deflecting apparatus 7. Further, before the laser beams LYa,LMa, LCa and LBa respectively emitted from the lasers 3Ya, 3Ma, 3Ca and3Ba constituting the respective light sources are combined with therespective laser beams LYb, LMb, LCb and LBb constituting a pair by thehalf mirrors 15Y, 15M, 15C and 15B, the reflecting angles of thecorresponding galvano mirrors 18Y, 18M, 18C and 18B are set topredetermined angles, whereby an interval in a sub scanning direction isset to a predetermined interval.

[0041] The pre-deflection optical system 5 includes a finite focal lens13 which applies a predetermined focusing characteristic to the laserbeam L emitted from the laser 3, an aperture stop 14 which applies anoptional cross sectional beam shape to the laser beam L passing throughthe finite focal lens 13, a half mirror (the first combining mirror) 15and a cylinder lens 17 which further applies a predetermined focusingcharacteristic to the laser beam L combined by the half mirror 15 withrespect to the sub scanning direction, and aligns the cross sectionalbeam shape of the laser beam L emitted from the laser 3 to apredetermined shape so as to guide to the reflection surface of theoptical deflecting apparatus 7. In this case, for the finite focal lens13, there is used a lens, for example, obtained by adhering ultraviolethardened type plastic lenses (not shown) onto at least one surface of alaser incidental surface and an emitting surface of a non-sphericalglass lens or a spherical glass lens (or integrally forming plasticlenses (not shown)). Further, the laser 3, the finite focal lens 13 andthe aperture stop 14 are integrally held by the lens holder 11 describedbelow with reference to FIG. 3.

[0042] As shown in FIG. 3 (which representatively shows the lens holder11 corresponding to the optional laser 3), the lens holder 11 is, forexample, made of an aluminum die casting having a high processingaccuracy and on the contrary having a small shape change with respect tothe change of the temperature, and is arranged on a recess portion 10 aof a base plate 10 for holding the elements of the pre-deflectionoptical system 5 in such a manner as to move along a direction of anarrow X on the recess portion 10 a. In this case, the base plate 10 ispositioned on a middle base la of the exposure apparatus 1.

[0043] The lens holder 11 has a holder main body 11 a which holds thelaser 3 fixed to an aluminum die casting laser holder 12 formed by analuminum substantially equal to the material of the lens holder 11 andthe finite focal lens 13 while maintaining them at a predeterminedinterval, and holds the finite focal lens 13 at a position apredetermined distance apart from a light emitting point of the laser 3,that is, a position in which the laser holder 12 and the holder mainbody 11 a are brought into contact with each other. That is, the finitefocal lens 13 is a lens with a cylindrical flange in which a flangeportion is formed in a cylindrical shape, and is fixed to the lensholder 11 by being pressed to a side surface 11 c of the lens holder bya plate spring 16 arranged in such a manner as to be pressurized towardthe side surface 11 c of the holder main body 11 a from a side portionof a bottom portion 11 b in the holder main body 11 a of the lens holder11. Accordingly, the finite focal lens 13 can move the holder main body11 a along an optical axis o directing toward the cylinder lens 17 afterpassing through the finite focal lens 13 from the laser 3, and is fixedto the lens holder 11 so that the interval with respect to the laser 3fixed to the laser supporting body 12 becomes a predetermined interval.

[0044] Again with reference to FIGS. 1 and 2, the half mirror 15 (Y, M,C and B) is structured such that, for example, a metal membrane isvacuum evaporated on one surface of a parallel flat glass formed so asto have a thickness t=5 mm, whereby a transmittance and a reflectivityare controlled to a predetermined magnitude, and the reflecting anglesof the main scanning direction and the sub scanning direction arerespectively set by a mirror holding mechanism 20 described below withreference to FIG. 4 on the basis of horizontal synchronism and beamposition signals obtained by detecting one laser beam, for example, thelaser beams L (Y, M, C and B)a or L (Y, M, C and B)b among two laserbeams emitted from the respective lasers 3 (any one of Y, M, C and B)aand the lasers 3 (any one of Y, M, C and B)b by the horizontalsynchronism detector 23.

[0045] In this case, the respective laser beams LYa, LMa, LCa and LBaare respectively transmitted through the half mirrors 15 (Y, M, C and B)as mentioned above, and the respective laser beams LYb, LMb, LCb and LBbemitted from the lasers 3Yb, 3Mb, 3Cb and 3Bb are reflected by the halfmirrors 15 (Y, M, C and B). further, a number at which the respectivelaser beams L (Ya, Yb, Ma, Mb, Ca, Cb, Ba and Bb) transmit through thehalf mirrors 15 (Y, M, C and B) is 1 or 0 as mentioned above.Specifically, LBa, LMa, LCa and LYa transmit through the half mirrors 15(Y, M, C and B) at only one time, and the other laser beams LBb, LMb,LCb and LYb are reflected by the half mirrors 15 (Y, M, C and B). Inthis case, the respective half mirrors 15 (Y, M, C and B) are inclinedin the same direction as that of the laser beams LBa, LMa, LCa and LYaafter transmitting through the respective half mirrors 15 (Y, M, C andB) and moving toward the optical deflecting apparatus 7 at the sameamount (angle). In this case, angles U at which the respective halfmirrors 15 (Y, M, C and B) are inclined are respectively set to 45degrees.

[0046] In this case, it is possible to set the outputs of the laserelements 3Ya and 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb of therespective light sources 3 (Y, M, C and B) to be substantially the sameby setting a ratio between the reflectivity and the transmittance of therespective half mirrors 15 (Y, M, C and B). Accordingly, it is possibleto make the output on the image forming surface the same value, and itis easy to unify the image forming characteristic of the respectivelaser beams L (Ya, Yb, Ma, Mb, Ca, Cb, Ba and Bb).

[0047]FIG. 4 (representatively showing an optional laser beam) is aschematic view explaining a mirror holding mechanism 20 which can adjustan incline of a light incidental surface and a light emitting surface(light reflecting surface) of the half mirrors 15 (Y, M, C and B) forcombining three pieces×two groups lasers 3 (any one of Y, M, C and B)aand the lasers 3 (any one of Y, M, C and B)b constituting a pair(together with the lasers 3 (any one of Y, M, C and B)a in a directionwith respect to each of the main scanning direction and the sub scanningdirection.

[0048] As shown in FIG. 4, the half mirror 15 is fixed to apredetermined position of the base plate 10 by a projection-like mirrorholding portion 10 b integrally formed with the base plate 10 and aplate spring 20 a arranged in such a manner as to be pressurized towardthe mirror holding portion 10 b so as to indicate an optional inclinewith respect to the optical axis o.

[0049] The mirror holding mechanism 20, in particular, has a firstadjusting screw 20 b provided in a side near the bottom portion of themirror holding portion 10 b, that is, the base plate 10, and second andthird adjusting screws 20 c and 20 d provided in a portion apredetermined distance apart from the base plate 10, and it is possibleto set an incline of the mirror 15 pressed by the pressing force fromthe plate spring 20 a to a direction and an angle set according tofeeding amounts of three screws 20 b, 20 c and 20 d by independentlysetting the feeding amounts of the respective screws 20 b, 20 c and 20d. In this case, the plate spring 20a is separated into two web-likeareas except the portion fixed to the base plate 10 so as to be broughtinto contact only with an outer peripheral portion of the mirror 15, andthe mirror holding portion 10 b is structured such as to be notchedexcept an area in which 20 b, 20 c and 20 d are provided, thereby makingit possible to input or reflect the laser beam from both of the mirrorholding portion 10 b and the plate spring 20 a.

[0050] Next, a description will be in detail given of each of the laserbeams LYa and LYb, LMa and LMb, LCa and LCb, and LBa and LBb guided tothe reflecting surface of the optical deflecting apparatus 7 anddeflected (scanned) on the reflecting surface and the post-deflectionoptical system 9 positioned between the optical deflecting apparatus 7and the image surface at every lasers.

[0051] The laser beams LYa and LYb guided to the optical deflectingapparatus 7 are deflected according to rotation of the respectivereflecting surfaces of the polyhedral mirror 7 a in the opticaldeflecting apparatus 7 at a substantially constant speed, and input tothe post-deflection optical system 9, that is, the incidental surface ofthe first image forming lens 21 a in the two-assembled image forminglens 21 at a predetermined angle.

[0052] In the below description, the laser beams LYa and LYb are appliedpredetermined focusing characteristic and directivity by the secondimage forming lens 21 b so that the shape and the magnitude of the beamspot on the surface of the photosensitive drum (not shown) becomepredetermined shape and magnitude, successively reflected by the mirrors33Y and 35Y and reflected by the mirror 37Y at a predetermined angle,thereby passing through the dust-proof glass 39Y so as to be irradiatedonto the photosensitive drum (image surface) (not shown).

[0053] Similarly, the respective laser beams LMa, LMb, and LCa and LCbare passed through the second image forming lens 21 b, successivelyreflected by the mirrors 33M, 33C, 35M and 35C and reflected by themirrors 37M and 37C at a predetermined angle, thereby passing throughthe dust-proof glasses 39M and 39C so as to be irradiated onto thephotosensitive drum (not shown).

[0054] The laser beams LBa and LBb are applied predetermined focusingcharacteristic and directivity by the second image forming lens 21 b inthe same manner as that of the laser beam corresponding to the othercolors mentioned above, and reflected only by the mirror 33B at apredetermined angle, thereby passing through the dust-proof glass 39M soas to be irradiated onto the photosensitive drum (not shown).

[0055] In this case, the third mirrors 37 (Y, M and C) provided incorrespondence to the respective laser beams L (Y, M and C) are held bya parallelism adjusting mechanism (not shown) in such a manner as toreflect the laser beams L (Y, M and C) in an optional direction, and areformed in such a manner as to set a changing amount of the beam spotdiameters at both end portions in the longitudinal direction of theimage surface to an optional magnitude.

[0056] Next, a description will be in detail given of characteristics ofthe laser beams (Y, M, C and B) guided to the photosensitive drum (theimage surface) (not shown) from the multi-beam exposure apparatus 1mentioned above.

[0057] The laser beam LYa emitted from the yellow No. 1 laser 3Ya isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Ya, and passes through the aperture stop 14Ya, whereby apredetermined cross sectional beam shape is applied thereto.

[0058] The laser beam LYa to which the predetermined cross sectionalbeam shape is applied after passing through the aperture stop 14Ya isreflected to a predetermined direction by the semi-stationary mirror 18Yhaving a reflecting surface capable of being set to an optionaldirection, and guided to the half mirror 15Y. In this case, thesemi-stationary mirror 18 is a galvano mirror (a mirror held in such amanner as to move at a very small amount due to a power force) in whichan angle of the reflecting surface can be set in an optional directionby a fixing apparatus similar to the mirror holding mechanism 20 alreadyexplained with reference to FIG. 5 or an ultrasonic motor (not shown).

[0059] The laser beam LYa guided to the half mirror 15Y transmitsthrough the half mirror 15Y, is overlapped with the laser beam LYb fromthe yellow No. 2 laser 3Yb mentioned below by the half mirror 15Y, andis guided to the cylinder lens 17Y as the laser beam LY. The laser beamLY guided to the cylinder lens 17Y is further focused with respect tothe sub scanning direction by the cylinder lens 17Y and is guided to thepolyhedral mirror 7 a of the optical deflecting apparatus 7. In thiscase, the half mirror 15Y is arranged so that the reflecting angle inthe sub scanning direction becomes a predetermined angle with respect tothe laser beam LYa. Further, the incline in the sub scanning directionof the half mirror 15Y is set on the basis of the beam position dataobtained by the horizontal synchronism and sub scanning beam positiondetector 23 in the post-deflection optical system 9 mentioned below.

[0060] The laser beam LYb emitted from the yellow No. 2 laser 3Yb isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Yb, and passes through the aperture stop 14Yb, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LYb to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Yb is reflected by thehalf mirror 15Y, overlapped with the laser beam LYa from the yellow No.1 laser 3Ya mentioned above by the half mirror 15Y, and guided to thepolyhedral mirror 7 a of the optical deflecting apparatus 7.

[0061] The laser beam LMa emitted from the magenta No. 1 laser 3Ma isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Ma, and passes through the aperture stop 14Ma, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LMa to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Ma is guided to thehalf mirror 15M. The laser beam LMa guided to the half mirror 15Mtransmits through the half mirror 15M, is overlapped with the laser beamLMb from the magenta No. 2 laser 3Mb mentioned below by the half mirror15M, and is guided to the cylinder lens 17M as the laser beam LM. Thelaser beam LM guided to the cylinder lens 17M is further focused withrespect to the sub scanning direction by the cylinder lens 17M and isguided to the polyhedral mirror 7 a of the optical deflecting apparatus7. In this case, the half mirror 15M is arranged so that the reflectingangle in the sub scanning direction becomes a predetermined angle withrespect to the laser beam LMa. Further, the incline in the sub scanningdirection of the half mirror 15M with respect to the laser beamcorresponding to a reference in which the reflecting angle in the subscanning direction is set is set on the basis of the beam position dataobtained by the horizontal synchronism and sub scanning beam positiondetector 23 in the post-deflection optical system 9 mentioned below.

[0062] The laser beam LMb emitted from the magenta No. 2 laser 3Mb isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Mb, and passes through the aperture stop 14Mb, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LMb to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Mb is reflected by thehalf mirror 15M, overlapped with the laser beam LMa from the magenta No.1 laser 3Ma mentioned above by the half mirror 15M, and guided to thepolyhedral mirror 7 a of the optical deflecting apparatus 7.

[0063] The laser beam LCa emitted from the cyan No. 1 laser 3Ca isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Ca, and passes through the aperture stop 14Ca, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LCa to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Ca is guided to thehalf mirror 15C. The laser beam LCa guided to the half mirror 15Ctransmits through the half mirror 15C, is overlapped with the laser beamLCb from the cyan No. 2 laser 3Cb mentioned below by the half mirror15C, and is guided to the cylinder lens 17C as the laser beam LC. Thelaser beam LC guided to the cylinder lens 17C is further focused withrespect to the sub scanning direction by the cylinder lens 17C and isguided to the polyhedral mirror 7 a of the optical deflecting apparatus7. In this case, the half mirror 15C is arranged so that the reflectingangle in the sub scanning direction becomes a predetermined angle withrespect to the laser beam LCa. Further, the incline in the sub scanningdirection of the half mirror 15C corresponding to a reference in whichthe reflecting angle in the sub scanning direction is set is set on thebasis of the beam position data obtained by the horizontal synchronismand sub scanning beam position detector 23 in the post-deflectionoptical system 9 mentioned below.

[0064] The laser beam LCb emitted from the cyan No. 2 laser 3Cb isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Cb, and passes through the aperture stop 14Cb, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LCb to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Cb is reflected by thehalf mirror 15C, overlapped with the laser beam LCa from the cyan No. 1laser 3Ca mentioned above by the half mirror 15C, and guided to thepolyhedral mirror 7 a of the optical deflecting apparatus 7.

[0065] The laser beam LBa emitted from the black No. 1 laser 3Ba isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Ba, and passes through the aperture stop 14Ba, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LBa to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Ba is reflected to apredetermined direction by the semi-stationary mirror 18B having areflecting surface capable of being set to an optional direction, andguided to the half mirror 15B.

[0066] The laser beam LBa guided to the half mirror 15B transmitsthrough the half mirror 15B, is overlapped with the laser beam LBb fromthe black No. 2 laser 3Bb mentioned below by the half mirror 15B, and isguided to the cylinder lens 17B. The laser beam LB guided to thecylinder lens 17B is further focused with respect to the sub scanningdirection by the cylinder lens 17B and is guided to the polyhedralmirror 7 a of the optical deflecting apparatus 7. In this case, the halfmirror 1BB is arranged so that the reflecting angle in the sub scanningdirection becomes a predetermined angle with respect to the laser beamLBa. Further, the incline in the sub scanning direction of the halfmirror 15B corresponding to a reference in which the reflecting angle inthe sub scanning direction is set is set on the basis of the beamposition data obtained by the horizontal synchronism and sub scanningbeam position detector 23 in the post-deflection optical system 9mentioned below.

[0067] The laser beam LBb emitted from the black No. 2 laser 3Bb isconverted into a direction substantially parallel to each of the mainscanning direction and the sub scanning direction by the finite focallens 13Bb, and passes through the aperture stop 14Bb, whereby apredetermined cross sectional beam shape is applied thereto. The laserbeam LBb to which the predetermined cross sectional beam shape isapplied after passing through the aperture stop 14Bb is reflected by thehalf mirror 15B, overlapped with the laser beam LBa from the black No. 1laser 3Ba mentioned above by the half mirror 15B, and guided to thepolyhedral mirror 7 a of the optical deflecting apparatus 7.

[0068] In this case, the semi-stationary mirrors 18Y and 18B positionedon the optical path of the laser beam LYa emitted from the yellow No. 1laser 3Ya and the laser beam LBa emitted from the black No. 1 laser 3Baare arranged in such a manner as to change the reflecting direction andthe angle of the laser beam in each of the main scanning direction andthe sub scanning direction, for example, by the mirror holding mechanismsimilar to the mirror holding mechanism 20 for holding the half mirror15 shown in FIG. 4.

[0069] The 3 pieces×2 groups=6 pieces laser beams LM, LC and LB combinedby the half mirrors 15M, 11C and 11B correspond to the respective laserbeams L (M, C and B), and in the same manner as that of the half mirrors15 (Y, M, C and B) and the semi-stationary mirrors 18Y and 18B, arereturned to a predetermined direction by the color combining mirrors(that is, the second combining mirrors) 19M, 19C and 19B in such amanner as to change the reflecting direction and the angle of the laserbeam in each of the main scanning direction and the sub scanningdirection by the mirror holding mechanism similar to the mirror holdingmechanism 20 for holding the half mirror 15 shown in FIG. 4, so as to beguided to the optical deflecting apparatus 7.

[0070] In this case, the laser beam LY obtained by combining two laserbeams LYa and LYb by means of the half mirror 15Y is not reflected onthe middle, and is linearly guided toward the optical deflectingapparatus 7. That is, the laser beam LY passes through a space which isnot shielded by any mirrors disposed at a distance in a direction of therotary shaft of the reflecting surface of the polyhedral mirror 7 a inthe optical deflecting apparatus 7 with respect to each of the colorcombining mirrors 19M, 19C and 19B, so as to be guided to the opticaldeflecting apparatus 7.

[0071] Then, four colors=four pieces laser beams L combined into onepiece by the pre-deflection optical system 5 mentioned above, guided tothe optical deflecting apparatus 7 and deflected (scanned) on each ofthe reflecting surfaces of the optical deflecting apparatus 7 areapplied a predetermined characteristic by the post-deflection opticalsystem 9, and are image formed at a predetermined position on therespective photosensitive drum 58.

[0072] In this case, according to any one of a time except writing theimage, for example, a time before beginning to write the image dataafter the image forming apparatus is started, or a time on the way ofcontinuously forming the image, or a timing that printing does notaffect on the sheet by the scanning optical system, or a fixed timeinterval, or any optional combination thereof, with respect to the subscanning direction, a distance between the laser beams comprising a pairwhich are guided to the respective image forming portion, that is, therelative positional relations between the LYa and LYb, LMa and LMb, LCaand LCb, and LBa and LBb are measured, and on the basis of the measuredresults, the respective laser beam positions and the reflecting anglesof the galvano mirrors 18Y, 18M, 18C and 18B are controlled so that therelative positional relations thereof become a predetermined interval.Further, also with respect to the main scanning direction, according toany one of a time except writing the image, for example, a time beforebeginning to write the image data after the image forming apparatus isstarted, or a timing on the way of continuously forming the image andthat printing does not affect on the sheet by the scanning opticalsystem, or a fixed time interval, values obtained by measuring adistance between the laser beams which are guided to the respectiveimage forming portion, that is, the relative passing timing between theLYa and LYb, LMa and LMb, LCa and LCb, and LBa and LBb are kept, and onthe basis of the measured results, the light emitting timing of thelight sources 3Ya and 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb iscontrolled so as to cancel the difference of the passing timing.

[0073] Further, according to the result obtained by detecting thedifference of the image written by the respective image forming portions(not shown), the difference in the main scanning direction and thedifference in the sub scanning direction between the laser beams LY(=LYa+LYb), LM (=LMa+LMb), LC (=LCa+LCb) and LB (=LBa+LBb) are detectedby the resist sensors (not shown) of the image forming apparatus mainbody (not shown), so that the difference in the sub scanning directioncan be corrected according to the timing of writing the image and thedifference in the main scanning direction can be corrected according tothe timing and the image frequency of writing the image.

[0074] In this case, as is explained in the description of the priorart, the semiconductor laser elements are respectively different in thechanging amount of the emitting wavelength with respect to thetemperature change. In this case, when the temperature is varied in theperiphery of the respective laser elements or a difference is generatedin the level of the change with age in the respective laser elements,the wavelength of the light beams outputting from the respective lightsources is varied. Further, since there is a mode hopping phenomenonthat the emitting wavelength is about 1.5 nm changed with respect to thetemperature change about 0.1° C. as a characteristic of thesemiconductor laser elements, it is hard to uniformly align the emittingwavelength of all the laser elements at a wide environmental temperaturerange even when aligning the emitting wavelength under a certaincondition.

[0075] On the other hand, when the wavelength of the laser beam to beirradiated the laser component is changed, a difference of between theangles reflected by the lenses 21 a and 21 b of the laser beam (in whichthe wavelength is changed) and the laser beam having the referencewavelength is generated although the rotary angles of the respectivereflecting surfaces of the optical deflecting apparatus 7 are the same.

[0076] In many cases, a chromatic aberration of both ends of aneffective field angle of the respective lenses in the two-assembled lens21 of the post-deflection optical system 9 is hardly “0”, so that in thecase that the wavelength of the laser beam irradiated from the lasercomponent is changed, the laser beam successively passing through therespective lenses 21 a and 21 b of the two-assembled lens 21 in thisorder is input to the light detector for the horizontal synchronism 23at a timing different from the timing at which the laser beam having thereference wavelength is input.

[0077] However, by applying the characteristics shown below to theoptical path correcting element 27, in the case that the rotationalangles of the respective reflecting surfaces in the optical deflectingapparatus 7 are the same, it is possible to make the positions of thelaser beams on the light detecting surface of the light detector fordetecting the horizontal synchronism 23 substantially equal to eachother by changing the emitting angles of the laser beams irradiated fromthe respective laser elements 3Ya, 3Yb, 3Ma, 3Mb, 3Ca, 3Cb, 3Ba and 3Bbof the respective light sources 3 toward the light detecting surface ofthe light detector for detecting the horizontal synchronism 23 incorrespondence to the wavelengths of the laser beams.

[0078] In particular, by using the prism having a cross sectional shapeformed in an isosceles triangle shown in FIG. 5 for the optical pathcorrecting element 27, the wavelength of the laser beam emitted form thelight source is changed according to the change of the temperature,whereby the laser beams are irradiated to the different positions so asto be reflected although the respective reflecting surfaces of theoptical deflecting apparatus 7 have the same rotational angle, so thatit is possible to reduce an influence of the phenomenon that the beamsare actually guided to the different positions of the light detector 23for detecting the horizontal synchronism 23 at the same timing.

[0079]FIG. 6 is a graph showing a state in which the light emittingwavelength is changed when the environmental temperature is changedaccording to the mode hopping of the semiconductor laser element.

[0080] As shown in FIG. 6, it is recognized that the light emittingwavelength of the laser beam irradiated from a certain semiconductorlaser element is about 2 nm lengthened (the oscillating frequency isreduced) as the environmental temperature (in this case, the temperatureof the case surrounding the light emitting chip of the laser element)increases at 10° C.

[0081] However, as shown in A portion and B portion in FIG. 6, thechange of the temperature and the wavelength are locally nonlinear, andas already explained, there is a case that the wavelength is 1 nm ormore changed even when the temperature change is significantly small. Inthis case, the temperature at which the local wavelength change isgenerated is different at every laser element units, and it can not bedefined at the current stage.

[0082]FIG. 8 is a graph showing a change of a position of a laser beamimage formed on the image surface after passing through the respectivelenses 21 a and 21 b of the two-assembled lens 21 as a relative positionin the main scanning direction in the case that the wavelength of thelaser beam after emitting the laser component is changed, in order tospecify a characteristic of the prism (the optical path correctingelement) 27 shown in FIG. 5.

[0083] As shown in FIG. 8, with reference to the laser beam having awavelength of 680 nm (a curve a), it is recognized that the imageforming positions of the laser beams having wavelength of 665 nm (acurve b), 670 nm (a curve c), 675 nm (a curve d), 685 nm (a curve e),690 nm (a curve f) and 695 nm (a curve g) in the main scanning directionare about 0.045 mm changed at the maximum in connection with the changeof the oscillating angles of the respective reflecting surfaces of theoptical deflecting apparatus 7. In this case, as shown in FIG. 8, apolarity that the image forming position is changed becomes an oppositedirection. Further, as shown in FIG. 6, the laser elements frequentlygenerate the local wavelength change, and accordingly, the magnitude ofthe relative value shown in FIG. 8 actually includes the greaterchanging elements.

[0084]FIG. 9 is a graph which shows a change of the beam position imageformed on the image surface as a relative position in the main scanningdirection in the case that the wavelength of the laser beam from each ofthe laser components indicates the temperature-wavelength change shownin FIG. 8 in a state of taking out the optical path correcting element27 from the multi-beam exposure apparatus 1 corresponding to theembodiment according to the present invention shown in FIGS. 1 and 2. Inthis case, in FIG. 9, respective curves α to ζ respectively showdifferences at every conditions in which the wavelengths are 5 nmdifferent, that is, a difference between the wavelengths of 665 and 670(a curve α), a difference between the wavelengths of 670 and 675 (acurve β), a difference between the wavelengths of 675 and 680 (a curveγ), a difference between the wavelengths of 680 and 685 (a curve δ), adifference between the wavelengths of 685 and 690 (a curve ε), adifference between the wavelengths of 690 and 695 (a curve ζ). In thiscase, also in FIG. 9, in the case of generating the local wavelengthchange shown in FIG. 6, it is further greater changed.

[0085] In this case, as shown in FIG. 10, in the case of setting adirection that the respective reflecting surfaces of the opticaldeflecting apparatus 7 are rotated to a minus (−) direction from a plus(+) direction and setting the respective laser beams to be moved on theimage surface from the minus (−) direction to the plus (+) direction, itis recognized that a rotational angle Δθ of each of the reflectingsurfaces in the optical deflecting apparatus 7 is Δθ=7.5 μrad shifterbetween the timing of detecting the laser beam having the wavelength of675 nm and the timing of detecting the laser beam having the wavelengthof 680 nm when setting the position at which the light detector fordetecting the horizontal synchronism 23 is provided to −160 mmcorresponding to the position in the main scanning direction. This factshows the same motion as that in the case that the position at which thelight detector for detecting the horizontal synchronism 23 is providedis about 7.5 nm moved to the plus (+) side in the main scanningdirection.

[0086] Further, an apparent difference of the detector for thehorizontal synchronism 23 mentioned above just corresponds to writingthe image in a state of being Δθ=7.5 μrad shifted in all the image area,so that the relative positional shifting amount of the writing startposition between two laser beams having different wavelengths becomes anaffected component shifting at the rotational angle Δθ of the respectivereflecting surfaces in the optical deflecting apparatus 7 generated forsetting the writing timing reference to the main scanning position atthe end portion having the different position according to thewavelength, with respect to the shifting amount 7.5 μm of the beamposition expressed by 0.0675−0.0680 shown in FIG. 9, that is, theposition in the main scanning direction y is expressed by the formulay=y+Δy, and the magnitude becomes a value obtained by adding Δy=7.5 μm.

[0087] Accordingly, as shown in FIG. 11, the positions are substantiallyequal to each other near the position that the position in the mainscanning direction y satisfies the formula y=−160, however, near theopposite side which satisfies the formula y=160, the total 15 μmdifference is generated by adding the position shifting mount in themain scanning direction of the beam at the horizontal synchronousposition Δy=7.5 μm to the shifting amount of the beam position 7.5 μm.

[0088] Again, with reference to FIG. 5, the prism 27 formed in anisosceles triangle corresponds to an optical component which can returnthe amount that the writing position is shifted to the main scanningdirection according to the wavelength change of the laser beam due tothe temperature shown in FIGS. 6 to 10 and can input to the position atwhich the laser beam having the wavelength of the reference on thedetecting surface of the light detector for detecting the horizontalsynchronism 23 provided at the predetermined position is input, and moreparticularly, when setting the top angle of the prism 27 to “A”, arefractive index of the prism to “n”, and an angle formed between theincidental laser beam and the emitting laser beam when the laser beamhaving a wavelength of λ is input at an incident angle α, that is, anangle of deflection to B, the following formulas are established.

sin α=n sin (A/2)  (1)

[0089] And the formula (3) is introduced from the formula (2).

(B+A)/2=α  (2)

n=sin ((B+A)/2)/ sin (A/2)  (3)

[0090] In accordance with the formulas (1) to (3), when setting thewavelength of the reference laser beam to X and the wavelength of thelaser beam changing in corresponding to the temperature change to λ+λΔ,in comparison with the time when the laser beam having the wavelength ofλ is input to the prism 27 at the incident angle α, the changing amountΔB of the angle of deflection B and the changing amount Δλ of thewavelength when the laser beam having the wavelength of λ+Δλ is inputcan be expressed by the following formula.

ΔB/Δλ

=ΔB/βn×Δn/Δλ

=2 sin (A/2)/(1−n² sin ² (A/2))^((½))×Δn/Δλ  (4)

[0091] Further, when setting the distance between the prism 27 and thelight detector for detecting the horizontal synchronism 23 to D, it ispossible to cancel the change of the laser beam in the two-assembledlens in the case that the wavelength λ of the laser beam is changedtoλ+Δλ by setting the shape and the position of the prism 27 so as tosatisfy the following formula.

Δy/Δλ

=−D×ΔB/Δλ

=−D×2 sin (A/2)/(1−n² sin ²(A/2))^((½)×)

Δn/Δλ   (5)

[0092] In this case, Δy/Δλ in the formula (5) can be determined bycalculating each of the position at which the laser beam having thewavelength of λ is input and the position at which the laser beam havingthe wavelength of λ+Δλ is input on the detecting surface of the detectorfor detecting the horizontal synchronism 23 after taking out the prism(optical path correcting element) 27 of the multi-beam exposureapparatus 1 shown in FIGS. 1 and 2, on the basis of the characteristicsof the respective lenses 21 a and 21 b of the two-assembled lens 21, andsetting the results to y sns and y sns+Δy.

[0093] Accordingly, on the basis of the formula (5), it is sufficient toset D, A, n and Δn. In this case, since n and Δn are defined by amaterial of the glass utilized for the prism 27, the range of D and A inwhich the prism 27 can be arranged can be set when the material of theglass is determined.

[0094] Here, in the case of previously determining D and calculating A,the following formula can be introduced by solving the formula (5) withrespect to A.

A=2 arc sin ((Δy/Δλ)/(4D²(Δn/Δλ)²+

n²(Δy/Δλ)2)(^(½)))  (6)

[0095] The incident angle and the emitting angle α in this case can becalculated by the following formula.

[0096] a=arc sin (n sin (A/2))  (7)

[0097] More particularly, when taking out the prism 27 from themulti-beam exposure apparatus 1 shown in FIGS. 1 and 2, the laser beaminput to the light detector for detecting the horizontal synchronism 23is guided to the 7.5 μm shifting position as shown in FIG. 8 accordingto the change of the wavelength λ of 5 nm, so that the following formulacan be obtained on the basis of the formula (5).

Δy/Δλ

=7.5×10⁻³/5

=1.5×10⁻³ (mm/nm)  (8)

[0098] At this time, when setting the material of the prism 27 to BK 7(the optical glass), the refractive index n and the refractive indexchange/the wavelength change Δn/Δλ can be expressed by n=1.513605 andΔn/Δλ=−2.8486×10⁻⁵ (1/nm).

[0099] Further, when setting the distance between the prism 27 and thelight detector 23 for detecting the horizontal synchronism 23 to 123 mm,the following formula can be obtained by the formula (6) and D and A canbe calculated.

A=2 sin ⁻² ((1.5×10⁻³)/(4D 2(−2.8486×10⁻⁵)² +

1.513605²(1.5×10⁻³)²)^((½)))  (9)

[0100] Hereinafter, there are shown in Table 1, a position of the prism27 optimized on the basis of the result of pursuing the beam accordingto a computer simulation, a distance from the position of the laser beamin the sub scanning direction and the light detector for detecting thehorizontal synchronism in the main scanning direction, a combination ofthe incident angle a with respect to the chief ray of the laser beamemitted from the second lens 21 b of the two-assembled lens 21, the topangle A and the incident angle α calculated from the formulas (6) and(7), and characteristics when setting the material of the prism 27 toBK7. In this case, in Table 1, x1, y1 indicate relative coordinates of acrossing point between the incident surface of the laser beam toward theprism 27 and the chief ray of the laser beam emitted from the secondlens 21 b when setting a crossing point between the optical axis of theemitting surface of the second lens 21 b of the two-assembled lens 21and the lens surface to the origin. Further, the respective parametersshown in Table 1, that is, θ1, θ2, D3, θ4, θ5, D6, θ7, y7 (with respectto θ1, θ2, D3, θ4, D6, θ7, y7, the positional relations are respectivelyshown in FIG. 7) show that the chief ray of the laser beam is input tothe position of −160 mm when the optical path correcting component, thatis, the prism 27 is structured such as to set the position at which therotational angle of the reflecting surface in the optical deflectingapparatus 7 is 0 to a center in the main scanning direction, andrespectively indicate an angle formed with respect to the incidentsurface at the incident position, a corresponding (defined in FIG. 5)angle γ, a distance between the incident surface and the emittingsurface, an angle γ with respect to the emitting surface (defined inFIG. 5), an angle corresponding to the angle α shown in FIG. 5, adistance at which the chief ray emitted from the prism 27 is input tothe surface glass of the horizontal synchronism detector 23, an anglebetween the surface glass of the horizontal synchronism detector 23 andthe chief ray inputting to the surface glass of the horizontalsynchronism detector 23, and an amount at which the chief ray is movedin the main scanning direction by the surface glass of the horizontalsynchronism detector 23 having a thickness of 0.4 mm. In this case, asshown in Table 2, A corresponds to the top angle defined by using FIG.5, θ4 becomes substantially A/2 and θ5 becomes substantially α accordingto the formulas (6) and (7). TABLE 1 Shape, position and chief-ray pathof prism made of BK7 Results obtained by tracking rays x1 y1 θ1 θ2 D3 θ4−10 −79.0999 −35.6863 7.524531 −2.01905 7.524531 −15 −81.3526 −36.03827.754328 −2.08179 7.754328 −20 −83.6054 −36.4122 7.998374 −2.14867.998374 −25 −85.8581 −36.8105 8.258014 −2.21988 8.258014 −30 −88.1109−37.2354 8.534768 −2.2961 8.534768 −35 −90.3636 −37.6897 8.83034 −2.37788.83034 −40 −92.6164 −38.1765 9.146687 −2.46559 9.146687 −45 −94.8691−38.6992 9.486015 −2.56017 9.486015 −50 −97.1219 −39.2621 9.850863−2.66237 9.850863 −55 −99.3746 −39.8698 10.24413 −2.77315 10.24413 −60−101.627 −40.5278 10.66917 −2.89362 10.66917 −65 −103.88 −41.242411.12986 −3.02514 11.12986 −70 −106.133 −42.0212 11.63071 −3.169311.63071 −75 −108.386 −42.8728 12.17698 −3.32801 12.17698 −80 −110.638−43.8078 12.77485 −3.50361 12.77485 x1 θ5 D6 θ7 y7 −10 −11.4323 −178.52626.07101 −0.11995 −15 −11.7842 −173.003 26.41873 −0.12158 −20 −12.1583−167.477 26.78739 −0.1233 −25 −12.5565 −161.949 27.17892 −0.12512 −30−12.9815 −156.417 27.59562 −0.12707 −35 −13.4357 −150.882 28.04001−0.12915 −40 −13.9225 −145.344 28.51474 −0.13136 −45 −14.4453 −139.80229.02319 −0.13374 −50 −15.0082 −134.255 29.56901 −0.1363 −55 −15.6159−128.704 30.15651 −0.13905 −60 −16.2738 −123.147 30.79058 −0.14201 −65−16.9885 −117.583 31.47705 −0.14523 −70 −17.7672 −112.013 32.22249−0.14872 −75 −18.6189 −106.434 33.03508 −0.15253 −80 −19.5538 −100.84733.92406 −0.1567

[0101] TABLE 2 A obtained by approximation (6) α obtained by equation(7) A θ4-Equivalent = A/2 θ5-Equivalent = α 16.45229708  8.222614854−12.5075071 16.95068417  8.475342084 −12.8900459 17.47961375 8.739806875 −13.2963967 18.04191689  9.020958446 −13.728818218.64077876  9.320389378 −14.1898615 19.27978903  9.639894517−14.6824135 19.96301084  9.981505418 −15.2097571 20.68505114 10.34752557−15.775634 21.48115761 10.74057881 −16.3843312 22.32732313 11.16366157−17.0407775 23.24041855 11.62020927 −17.7506663 24.22834055 12.11417027−18.5205988 25.30021084 12.65010542 −19.3582766 26.46657293 13.23328647−20.2727083 27.73967404 13.86983702 −21.2745029

[0102] As explained above, it is possible to prevent the position atwhich the respective laser beams are projected on the image surface isshifted in the main scanning direction due to the variation of the lightemitting wavelengths caused by the change of the temperature between thelight sources 3Ya and 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb byinserting the prism 27 formed in an isosceles triangle shown in FIG. 5as the optical path correcting element for changing the direction of thelaser beam into the portion between the detecting surface of the opticaldetector for detecting the horizontal synchronism 23 and the reflectingmirror for detecting the horizontal synchronism 25 in correspondence tothe change of the wavelength due to the change of the temperature of thelaser beam emitted from the second lens 21 b of the two-assembled lens21 under a predetermined condition.

[0103] In this case, since the prism 27 shown in FIG. 5 has a functionof making an angle β times and a position 1/β times, in the case that βis not 1, that is, in the case that the focused beams are input, theimage forming position is shifted. For example, when the laser beamhaving a beam diameter h is input at a converging angle u, they form theimage at a position of 1=h/u from the place having the diameter h in thecase that the prism 27 does not exist, however, in the case that theprism 27 exist, since the beam diameter h satisfies the formula h=β×hand the converging angle u satisfies the formula u=u/β, the imageforming position 1 is expressed by the following formula.

1 ′=(β×h)/(u/β)=β² ×h/u=β² ×1   (10)

[0104] On the contrary, with respect to the sub scanning direction,since it is affected by unevenness of sensitivity on the detectingsurface of the light detector for detecting the horizontal synchronism23 or the change according to the shape of the edge portion when theposition of the laser beam is changed due to the change of thewavelength, it is necessary to make the incident surface and theemitting surface parallel to each other.

[0105] Further, since the laser beams guided to the image area from thesecond lens of the two-assembled lens 21 form image at the positionssubstantially equal to each of the main scanning direction and the subscanning direction in the case that the prism 27 does not exist, thereis generated a difference expressed by the formula of 1−1′=(1−β²)×1 atthe image forming position in the main scanning direction and the subscanning direction also on the detecting surface of the light detectorfor detecting the horizontal synchronism 23 positioned on the equivalentimage surface in the case that β is not 1, so that a great astigmatismis formed.

[0106] Accordingly, the image forming state becomes unstable in thedetecting portion such that a flare is easily generated, the beamdiameter is easily changed, and the like, so that the detecting accuracyis deteriorated.

[0107] In order to prevent the phenomenon from generating, it isnecessary to set β=1, and this is satisfied only in the case that theincident beam and the refracted beam become symmetrical with respect tothe top angle of the prism.

[0108] Accordingly, it is possible to restrict the generation of theflare or the change of the laser beam diameter, to stabilize the imageforming state on the detecting surface of the light detector 23 and toimprove a detecting accuracy by setting the incident beam and therefracted beam to a symmetrical relation with respect to the top angle Aof the prism 27 and setting the magnitude of the top angle A so that therespective laser beams in the main scanning direction and the subscanning direction are converged in the same place.

[0109] In this case, the prism 27 is, as the top angle A thereof isschematically shown in FIG. 1, arranged toward a direction in which thedistance between the reflecting position of the laser beam on thereflecting surface in the optical deflecting apparatus 7 and the secondlens 21 b becomes minimum when the laser beam in the direction of theimage area, that is, deflected by the optical deflecting apparatus 7 isinput to the second lens 21 b . This corresponds to the matter that thelaser beams passing through the first lens 21 a and the second lens 21 bof the two-assembled lens 21 are excessively refracted toward the centerof the main scanning direction of the lenses 21 a and 21 b in the casethat the wavelengths of the laser beams emitted from the respectivelaser elements become short.

[0110] As explained above, it is possible to guide the laser beam inwhich the wavelength is changed according to the change of thetemperature to the same position on the detecting surface of the lightdetector for detecting the horizontal synchronism in the case that thereflecting surface of the optical deflecting apparatus 7 has the samerotational angle, by using the prism 27 on the optical path between theimage forming lens 21 disposed between the optical deflecting apparatus7 and the image surface, and the light detector 23 for detecting thehorizontal synchronism 23, the prism being structured such as to changethe emitting angle in correspondence to the change of the wavelengthcaused by the change of the temperature of the laser beam from the lightsource with respect to the main scanning direction and to shift the beamposition at the same amount as the position shifting amount generated bythe image forming lens due to the difference of the wavelength and inthe direction opposite thereto.

[0111] Accordingly, it is possible to set the shifting of the printingposition in the main scanning direction at the opposite side to thewriting position in the main scanning direction generated by theshifting of the writing timing caused by the laser beam having adifferent wavelength from the reference wavelength to about half theshifting amount of the printing position in the main scanning directionwhich has been generated at the opposite side to the writing position inthe main scanning direction.

[0112]FIGS. 12 and 13 are schematic views which shows another embodimentof the multi-beam exposure apparatus shown in FIGS. 1 and 2. In thiscase, since the multi-beam exposure apparatus shown in FIGS. 12 and 13corresponds to the structure obtained by replacing the mirror fordetecting the horizontal synchronism 25 of the exposure apparatus shownin FIGS. 1 and 2 by a diffraction grating mentioned below and taking outthe prism 27 shown in the exposure apparatus shown in FIGS. 1 and 2, thesame reference numerals will be attached to the same elements and adetailed description will be omitted.

[0113] As shown in FIGS. 12 and 13, the diffraction grating (that is,the laser beam direction converting element) 29 is arranged on theoptical path between the second lens 21 b of the two-assembled imageforming lens 21 in the post-deflection optical system 9 and the lightdetector for detecting the horizontal synchronism 23.

[0114] In this case, the diffraction grating 29 has an incline in thesub scanning direction so that all the laser beams emitted from thesecond lens 21 b of the two-assembled lens 21 move toward the lightdetector for detecting the horizontal synchronism 23, and is structuredsuch that the incident angle and the emitting angle have the oppositedirections and the same formed angles in the case of viewing from anormal line with respect to the plane of all the grating, toward themain scanning direction. In this case, the grating of the diffractiongrating 29 is structured such that grooves are formed in the directionparallel to the sub scanning direction at a predetermined pitchdescribed below in the main scanning direction. Further, the diffractiongrating 29 guides all the laser beams toward the light detector fordetecting the horizontal synchronism 23 positioned on the equivalentimage surface according to reflection.

[0115] As explained with reference to FIGS. 6 to 9, in the multi-beamexposure apparatus, in the case of not inserting the diffraction grating29, since the position in the main scanning direction of the laser beamguided on the image surface is 7.5 μm shifted as the wavelength of thelaser beam irradiated from the laser element corresponding to the lightsource is 5 nm increased, the formula Δy/Δλ=7.5×10⁻³/5=1.5×10⁻³ (mm/nm)is established.

[0116] Further, since the reference wavelength of the laser beam is 680nm, λ=680×10⁻⁶, and as the kind of the diffraction grating 29, there isemployed a saw-tooth-shaped echelette grating in which the grating isprovided in parallel to the sub scanning direction and a cross sectionalshape in the direction perpendicular to the direction of the grating isformed as shown in FIG. 13.

[0117] Hereinafter, a description will be given in detail of acharacteristic of the grating of the diffraction grating 29.

[0118] In the diffraction grating, that is, the echelette grating 29,when setting a grating constant in the case that a parallel luminousflux is input to the diffraction grating 29 to a, an incident angle to ψand an angle of diffraction to ψ′, a diffraction efficiency becomesmaximum when direction of the reflected beam on the respective groovesurface and of the diffracted beam from the total of the grating surfaceare coincident with each other (when the relation ψ+ψ′=2θb isestablished). Further, when setting an angle (an angle of deflection)formed between the incident beam and the diffracted beam to (ψ−ψ′), thediffraction efficiency can be made maximum when the angle θb formed bythe inclined surface and the flat portion of the grating 29 satisfiesthe following formula (11).

θb= arc sin (λ/(2a)/ cos ((ψ−ψ′)/2)  ( 11)

[0119] Here, the following formula is established.

mλ=( sin ψ+sin ψ′)a

m=±1,±2. . .   (12)

[0120] Then, an angular dispersion Δψ′/Δλ can be calculated bydifferentiating the formula (12) according to the following formula.

Δψ′/Δλ

=1/a((1−((mλ/a)−sin ψ)²)^((½)))  (13)

[0121] Hereinafter, it is possible to calculate Δy/Δλ by taking out thediffraction grating 29 in the same manner as the case of using the prism27, calculating each of the position in which the laser beam having awavelength of λ is input and the position in which the laser beam havinga wavelength of λ+Δλ is input, on the detecting surface of the lightdetector for detecting the horizontal synchronism 23 on the basis of thecharacteristics of the respective lenses 21 a and 21 b of thetwo-assembled lens 21 and setting the results to y sns and y sns+Δy.

[0122] Here, when setting a distance between the diffraction grating 29and the light detector for detecting the horizontal synchronism 23 to D,it is possible to cancel the change of the laser beam position in thetwo-assembled lens in the case that the wavelength λ of the laser beamis changed to λ+Δλ by setting the shape and the inserting position ofthe diffraction grating 29 so that the following formula is satisfied.

Δy/Δλ

=−D×Δψ′/Δλ

=−D×1/a((1−((mλ/a)−sin ψ)²)^((½)))   (14)

[0123] In this case, ψ can be calculated according to the followingformula (15).

[0124] ψ=arc sin ((mλ/a)±(1−(D²/(Δy/Δλ)²a²))^(1/2))   (15)

[0125] Then, on the basis of the formula (15), a, m, D and ψ are set, ψ′is calculated on the basis of the ψ and the formula (12) and θb iscalculated on the basis of the formula (11), respectively. In this case,ψ′ can be rewritten by the following formula.

ψ′=arc sin ((mλ/a)−sin ψ)  (16)

[0126] There are shown in Table 3 below a distance between thediffraction grating 29 optimized on the basis of the result of pursuitof the beam according to the computer simulation and the light detector23 for detecting the horizontal synchronism, an incident angle to thediffraction grating 29, an emitting angle from the diffraction grating29 and a local angle θb of the incident surface of the diffractiongrating 29. TABLE 3 Properties of diffraction crating Distance (mm)Angle of Order m of Grating between grating incidence Diffraction angleDiffraction constant a and sensor (degree) (degree) θb (degree) 1 0.1 2086.26335828 −82.34017276 1.96159276 1 0.1 30 80.61070764 −78.465355111.072676264 1 0.1 40 76.07182806 −74.53618831 0.767819878 1 0.1 5071.73564116 −70.5308595 0.60239083 1 0.1 60 67.41763658 −66.423780530.496928025 1 0.1 70 63.03049032 −62.18369467 0.423397823 1 0.1 8058.50885457 −57.77075117 0.369051701 1 0.01 10 54.42673418 −48.191106383.117813899 1 0.015 20 30.22981527 −27.26684862 1.481483326 1 0.025 3038.84499004 −36.87098507 0.987002488 1 0.03 40 28.73782667 −27.266848620.735489024 1 0.035 50 18.92610661 −17.75331375 0.586396427 1 0.045 6028.24525841 −27.26684862 0.489204897 1 0.05 70 21.87737923 −21.040090310.418644462 1 0.055 80 14.87326712 −14.1415273 0.36586991

[0127] As explained above, also in the case of using the diffractiongrating, it is possible to prevent the position at which the respectivelaser beams are projected on the image surface is shifted in the mainscanning direction due to the variation of the light emittingwavelengths caused by the change of the temperature between the lightsources 3Ya and 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb.

[0128] That is, it is possible to guide the laser beam in which thewavelength is changed according to the change of the temperature to thesame position on the detecting surface of the light detector fordetecting the horizontal synchronism in the case that the reflectingsurface of the optical deflecting apparatus 7 has the same rotationalangle, by using the diffraction grating 29 on the optical path betweenthe image forming lens 21 disposed between the optical deflectingapparatus 7 and the image surface, and the light detector 23 fordetecting the horizontal synchronism 23, the prism being structured suchas to change the emitting angle in correspondence to the change of thewavelength caused by the change of the temperature of the laser beamfrom the light source with respect to the main scanning direction and toshift the beam position at the same amount as the position shiftingamount generated by the image forming lens due to the difference of thewavelength and in the direction opposite thereto.

[0129] Accordingly, it is possible to set the shifting of the printingposition in the main scanning direction at the opposite side to thewriting position in the main scanning direction generated by theshifting of the writing timing caused by the laser beam having adifferent wavelength from the reference wavelength to about half theshifting amount of the printing position in the main scanning directionwhich has been generated at the opposite side to the writing position inthe main scanning direction.

[0130] As explained above, in the multi-beam exposure apparatusaccording to the present invention, it is possible to guide the laserbeam in which the wavelength is changed according to the change of thetemperature to the same position on the detecting surface of the lightdetector for detecting the horizontal synchronism in the case that thereflecting surface of the optical deflecting apparatus 7 has the samerotational angle, by using the prism 27 on the optical path between theimage forming lens 21 disposed between the optical deflecting apparatus7 and the image surface, and the light detector 23 for detecting thehorizontal synchronism 23, the prism being structured such as to changethe emitting angle in correspondence to the change of the wavelengthcaused by the change of the temperature of the laser beam from the lightsource with respect to the main scanning direction and to shift the beamposition at the same amount as the position shifting amount generated bythe image forming lens due to the difference of the wavelength and inthe direction opposite thereto.

[0131] Further, according to the multi-beam exposure apparatus of thepresent invention, it is possible to guide the laser beam in which thewavelength is changed according to the change of the temperature to thesame position on the detecting surface of the light detector fordetecting the horizontal synchronism in the case that the reflectingsurface of the optical deflecting apparatus 7 has the same rotationalangle, by using the diffraction grating 29 on the optical path betweenthe image forming lens 21 disposed between the optical deflectingapparatus 7 and the image surface, and the light detector 23 fordetecting the horizontal synchronism 23, the prism being structured suchas to change the emitting angle in correspondence to the change of thewavelength caused by the change of the temperature of the laser beamfrom the light source with respect to the main scanning direction and toshift the beam position at the same amount as the position shiftingamount generated by the image forming lens due to the difference of thewavelength and in the direction opposite thereto.

[0132] Accordingly, it is possible to reduce the shifting of theprinting position in the main scanning direction of the laser beam atthe opposite side to the writing position to about half.

[0133] Accordingly, it is possible to provide a color image formingapparatus which can provide a color image with no color shifting, and ahigh speed image forming apparatus with no fading and no spread of aprofile.

1. A multi-beam exposure apparatus comprising: a plurality of light sources for irradiating light beams having predetermined wavelengths; pre-deflection optical means for applying a predetermined optical characteristic to said light beam irradiated from each of said light sources; deflection means for deflecting said light beam passing through said pre-deflection optical means to a first direction corresponding to a direction in which a rotatably formed reflection surface is rotated at a predetermined speed; image formation optical means for continuously image forming said light beams deflected in said first direction by said deflection means on an image surface; detecting means for detecting at least one of said light beams passing through said image formation optical means and outputting predetermined signals corresponding to the light beams; and optical elements arranged between said deflection means and said detecting means and changing an emission angle in correspondence to a change of a wavelength of the light irradiated from each of the plurality of light sources.
 2. A multi-beam exposure apparatus according to claim 1, wherein said optical elements include a prism having an angle formed by the incidental surface and the emitting surface except 0 degree in a cross section viewed from said first direction.
 3. A multi-beam exposure apparatus according to claim 2, wherein said incidental surface and said emitting surface of said prism are structured such that an angle between the incidental surface of said prism and said light beam in said first direction when said light beam is incident to said prism is equal to an angle formed between a light beam emitted from said prism in said first direction when said light beam is emitted from said prism and the emitting surface of said prism.
 4. A multi-beam exposure apparatus according to claim 2, wherein a cross section of said prism viewed from said first direction is an isosceles triangle in which an angle between said incidental surface and said emitting surface is set to a top angle and lengths from the top angle are set to be equal to each other.
 5. A multi-beam exposure apparatus according to claim 4, wherein said top angle of said prism is directed to a direction in which a distance between the reflecting point on said reflecting surface of said deflecting means and said image forming means becomes minimum when said light beam is incident to said image forming means.
 6. A multi-beam exposure apparatus according to claim 1, wherein said optical element is a diffraction grating in which gratings are arranged in said first direction at a predetermined interval.
 7. A multi-beam exposure apparatus according to claim 1, wherein said optical element is a diffraction grating in which a groove is formed in a direction perpendicular to said first direction.
 8. A multi-beam exposure apparatus according to claim 1, wherein said detecting means is defined at a distance optically equivalent to said image surface and is arranged in an area except the image area among said image surface in which at least one of said light beams passing through said image forming means reaches.
 9. A multi-beam exposure apparatus according to claim 1, wherein said optical element changes an emitting angle in accordance that wavelengths of the light beams irradiated from said respective light sources change in response to a change of a temperature.
 10. A multi-beam exposure apparatus comprising: a plurality of light sources; a first optical element for assembling light beams irradiated from said plurality of light sources to one light beam so as to give a predetermined characteristic; deflection means for deflecting said light beams supplied from said first optical element to a first direction corresponding to a direction in which a rotatably formed reflection surface is rotated at a predetermined speed; a second optical element extended out along said first direction and image forming said light beams deflected from said deflection means to a predetermined position so as to satisfy a function corresponding to a rotation of said reflection surface in said deflection means; detecting means arranged at a distance optically equivalent to a position at which said light beams passing through said second optical element reaches and in an area except an image area in which said light beam passing through said second optical element functions as an image and detecting at least one of said light beams passing through said second optical element so as to output a predetermined signal; and optical elements arranged between said second optical element and said detecting means, changing an emission angle in correspondence to a change of a wavelength of the light irradiated from said plurality of light sources due to a change of a temperature with respect to said first direction and shifting a position to which the light reaches to a direction having the same amount as a position shifting amount on said predetermined image surface generated by said second optical element due to a difference of the wavelength and having an opposite direction, thereby guiding the light having a wavelength which is changed due to a change of the temperature to the same position on the detecting surface of said detecting means in the case that the reflection surface of said deflection means has the same rotational angle.
 11. A multi-beam exposure apparatus comprising: a plurality of light sources for irradiating lights having a predetermined wavelength at a predetermined temperature; pre-deflection optical means for assembling lights irradiated from said light sources to one light beam so as to give a predetermined characteristic; deflection means for deflecting a group of lights emitted from said pre-deflection optical means to a first direction; an image formation lens extended out in said first direction and image forming the lights deflected by said deflection means on a predetermined image surface at a uniform speed; detecting means defined at a distance optically equivalent to said predetermined image surface, arranged at a position in which the lights passing through said lens reaches and in an area except an image area among said predetermined image surface and detecting the lights passing through said lens so as to output a predetermined signal; and optical elements arranged on an optical path between said lens and said detecting means, changing an emission angle in correspondence to a change of a wavelength of the light irradiated from said plurality of light sources due to a change of a temperature with respect to said first direction and shifting a position to which the light reaches to a direction having the same amount as a position shifting amount generated by said lens due to a difference of the wavelength and having an opposite direction, thereby reducing a difference of the image of a main scanning direction position at a position opposite to a writing position in said first direction generated by a difference of the writing timing caused by the light being different from a reference wavelength in the wavelength. 